Dry plasma etching is the preferred process for etching features on a silicon wafer having semiconductor integrated circuits partially developed in it. Typically, one or more planar layers are deposited over the previously defined substrate, and a layer of photoresist mask or a hard mask is deposited over the planar layers and patterned to leave apertures exposing portions of the planar layers. An etching gas admitted into the etching reactor is then excited into a plasma state, and it acts on the portions of the planar layers exposed by the mask to remove those exposed portions. The plasma etching process has proved to be very effective at defining extremely small features with low production of deleterious particles.
The field of plasma etching is typically divided among silicon etching, oxide etching, and metal etching. Each uses its preferred chemistry and presents its own problems. However, many problems are common among them, and the etching chambers dedicated to different ones of the uses tend to resemble each other. Such commonality of design offers an opportunity for savings.
The most prevalent use of metal etching is to define interconnects (and accompanying contacts or vias) in a layer of aluminum or aluminum alloy deposited over an interlayer dielectric. Once the generally planar aluminum layer has been deposited over the interlayer dielectric and into the contact or via holes, a photomask is deposited and defined over the aluminum layer. Then, an etching gas is admitted into the plasma etch chamber and excited into the plasma state. It has long been known that a chlorine-based chemistry is effective at etching aluminum. See, for example, U.S. Pat. No. 5,387,556 to Xiaobing et al. Gaseous hydrochloric acid (HCl) is the prototypical chlorine-based etchant. However, HCl is no longer considered the optimum aluminum etchant.
Aluminum quickly forms an overlying layer of a native oxide of alumina (Al.sub.2 O.sub.3) and related materials forming a residue over the metallic aluminum being etched. Alumina is a very stable material and resistant to reductive breakdown, even by HCl. For these reasons, a plasma etch of BCl.sub.3, often in conjunction with HCl or Cl.sub.2, is often used for etching aluminum and its alloys. Wang et al. in U.S. Pat. No. 5,219,485 use a similar chemistry to etch silicides in order to avoid residues from the silicide etch.
However, the use of a powerful etchant like BCl.sub.3 introduces a problem originating from the fact that the chamber is most economically made of aluminum, for example the alloy Al1606-T6. The seminal problem is that a chamber having an aluminum body and which is used for etching aluminum must balance the etching of the aluminum portion of the substrate against the etching of the chamber body. The physical integrity of the aluminum chamber is not as important as the fact that the etching of the aluminum chamber is likely to produce aluminum-based particles that deleteriously fall on the wafer and reduces its yield of functioning integrated circuits. That is, the chamber wall in a plasma reactor intended for aluminum etching must not be composed of raw aluminum.
For these reasons, it has been known to coat the wall of a plasma reactor for metal etching with an etch-resistant coating. Steger describes such an approach in U.S. Pat. No. 5,268,200 in which a protective coating of an electrically conductive hydrogen-containing layer is deposited on the aluminum wall. Another more typical approach is to coat the aluminum body with a surface layer of alumina. This surface coating of alumina is usually achieved by anodization of the underlying aluminum. Raw aluminum quickly forms with a native oxide of Al.sub.2 O.sub.3 to thickness of about 2.5 nm. However, further increases in the oxide thickness are quickly inhibited by the robustness of the aluminum oxide layer. Electrolytic anodization of the aluminum body easily increases the alumina thickness to 25 to 75 .mu.m. Anodization of aluminum and aluminum-based alloys is well known. Typically, the aluminum body is submerged in a bath of electrolyte, for example, of 15 vol % solution of H.sub.2 SO.sub.4, and the aluminum body is connected as an anode to one terminal of an electrical power supply while a cathode submersed in the electrolyte is connected to the other terminal. Thereby, the aluminum is electrolytically oxidized by applying DC current. The first layer of a few tens of nanometers of anodization presents a relatively dense barrier. However, further increases in the anodization thickness produces a relatively porous material. Pore size can be reduced by reducing the temperature of the anodization bath, but inevitably the thicker anodizations lack the robustness of a native aluminum oxide layer or the initial barrier layer.
Anodized aluminum has been an object of much development for its use in plasma reactors, particularly metal etch reactors. The fundamental objective has been to reduce the etching of the anodized aluminum chamber wall relative to the etching of the alumina-based residues resulting from the etching of the aluminum lines. Etching of the anodized aluminum wall in a metal etch reactor is a particular problem since anodized aluminum is fundamentally alumina and BCl.sub.3 is being used for its effective removal of alumina.
The reaction of BCl.sub.3 and A1.sub.2 O.sub.3 is exothermic following the reaction EQU Al.sub.2 O.sub.3 +2BCl.sub.3 .fwdarw.2AlCl.sub.3 .Arrow-up bold.+B.sub.2 O.sub.3, .DELTA.G.sup.o.sub.f (-15.19 kcal/mol), (1)
where .DELTA.G.sup.o.sub.f is the reaction Gibbs free energy and the stated value is at 100.degree. C.
A first observation has been that anodized aluminum tends to be relatively light and porous. A denser alumina layer would perhaps be more resistant to etching relative to a residue on the aluminum being etched and would further reduce water uptake in the alumina, which complicates its use inside a vacuum chamber. Many attempts have been made to improve the quality of the anodized layer. One such approach uses hot deionized (DI) water in the anodization process so as to seal the anodized layer with a layer of boehmite (AlOOH),which forms according to the reaction EQU Al.sub.2 O.sub.3 +H.sub.2 O.fwdarw.2AlOOH.arrow-down dbl., .DELTA.G.sup.o.sub.f (-468.68 kcal/mol). (2)
Boehmite is chemically stable in the presence of BCl.sub.3, as seen by the reaction EQU 2AlOOH+2BCl.sub.3 .fwdarw.2AlCl.sub.3 +B.sub.2 O.sub.3 +H.sub.2 O, .DELTA.G.sup.o.sub.f (+398.03 kcal/mol), (3)
but boehmite is not a hard material and is not stable due to dehydration. While the reaction is useful for filling the pores of the anodized aluminum, the hot deionized water sealing after anodization is insufficient for the present needs.
The requirements for a corrosion-resistant coating have intensified recently with the introduction of plasma etch reactors utilizing high-density plasmas. High-density plasma (HDP) reactors have been recently developed for a number of processes. Generally, a high-density plasma is defined as one having an ionized plasma density of greater than 10.sup.11 cm-.sup.-3. An example of an HDP metal-etch reactor is the Decoupled Plasma Source (DPS) Metal Etch Chamber available from Applied Materials, Inc. of Santa Clara, Calif. Tepman et al. have described an early version of the DPS reactor chamber in U.S. patent application Ser. No. 08/296,043, filed Aug. 23, 1994 and incorporated herein by reference. The corresponding European application has been published as European Patent Application, 698,915-A1. The mechanical structure has changed somewhat in the commercial version of the DPS chamber.
A schematic representation of the commercial DPS chamber is illustrated in the cross-sectional view of FIG. 1. A upper, main processing compartment 10 is bounded by a curved ceramic dome 12, an upper housing 14 to which the ceramic dome 12 is sealed, and a movable pedestal wall 16 that is vertically movable to engage and seal within an inwardly extending annular shelf 18 of the upper housing 14. The upper housing 14 rests on and is sealed to a lower housing 20, and a bellows 22 is sealed to the bottom of the lower housing 20 and to a stem 24 extending downwardly from the pedestal wall 16. An electrode 19 may be included at the center of the dome 12. A lower compartment 26 is defined generally by the walls of the lower housing 20 and the lower edge of the annular shelf 18. During plasma processing, the movable pedestal wall 16 seals the upper compartment 10 from the lower compartment 22 by engaging and sealing the annular shelf 18 of the upper housing 14.
A vertical actuator 28 connected to the bottom of the stem 24 can move the pedestal wall 16 into and out of engagement with the annular shelf 18. An unillustrated robot blade can transfer a wafer 30 into the lower compartment through a loadlock slit 32 in the lower housing 20 and its unillustrated slit valve when the vertical actuator 28 has lowered the pedestal wall 16 to a position to receive the wafer 30 on its upper surface. The pedestal wall 16 typically includes an electrostatic chuck to selectively hold the wafer 30 by electrostatic attraction exerted by an electrical signal applied to the chuck. After the wafer has been deposited on the pedestal wall 16, the vertical actuator 28 raises the pedestal wall 16 so that it seals the upper compartment 10 and places the wafer within the upper compartment 10.
The upper housing 14 also includes a turbo port 38 connecting to an integral pumping stack 40. A vacuum pumping system 42 mated with the bottom of a pumping stack 40 pumps the upper compartment 10 as well as the lower compartment 26 when it is opened to the upper compartment 10. A poppet valve 44 fixed to the upper housing 14 over the pumping stack 40 can selectively isolate the upper compartment 10 from the vacuum pumping system 42.
Processing gas, which for aluminum etching typically includes BCl.sub.3, is injected into the sealed upper compartment 10 through a plurality, typically four, of unillustrated gas nozzles fixed to the radially inner ends of respective gas orifices 46 penetrating the upper housing 14 near its top. RF power is applied to an inductive coil 48 wrapped around the curved dome 12 so as to create a high-density plasma of the processing gas within the upper compartment 10. RF power is also applied to the pedestal wall 16 and possibly to a counter electrode fixed in the curved dome 12 so as to bias the plasma to effect the desired etching of the wafer.
The upper housing 14, as more clearly illustrated in the perspective view of FIG. 2, is a large, complexly shaped part having portions that are relatively inaccessible. As long as it can be formed of aluminum alloys, its manufacturing cost can be kept low, despite its complex shape, and it is relatively easy to anodize because anodization is a wet process.
However, it has been found that the use of a high-density BCl.sub.3 plasma intensifies the problem with chamber walls of anodized aluminum. A relatively high alumina etch rate has been observed in a circumferential band of the chamber wall around the gas jets and around the upper portion of the turbo port 38. The alumina etch is so strong in this band that with normal anodizations, the anodization is eroded away in spots within a week of steady use. At that point, a major portion of the chamber needs to be replaced.
We performed tests to determine quantitative erosion rates on coupons 49, as illustrated in FIG. 3, of anodized aluminum placed into the DPS chamber at various places inside the dome 12, on the chamber wall 14 near the turbo port 38, and, although not illustrated, on the pedestal 16 atop an aluminum nitride dummy wafer 30. The chamber was run with a standard commercial recipe including BCl.sub.3, Cl.sub.2, and Ar for 360 minutes with the RF turned on for 240 minutes of this time. The listed numbers are anodization erosion rates in micrometers per hour. The maximum erosion occurred at the top of the chamber wall adjacent to the turbo port 38, at which point the anodization erosion rate was measured to be 3.31 .mu.m per hour, an unacceptably high rate.
It has been found that the alumina etch rate in a high-density BCl.sub.3 plasma can be reduced by including some nitrogen in the etch processing gas. A series of tests used a gas flow having active components of about 90 sccm of Cl.sub.2, about 75 sccm of BCl.sub.3, and about 40 sccm of Ar in a high-density plasma reactor. Without any nitrogen, anodization erosion rates of up to between 0.87 to 1.4 .mu.m/hr were measured. However, with the addition of between 5 to 20 sccm of N.sub.2, the erosion rate was reduced to between 76 to 130 nm/hr.
However, the addition of nitrogen to the etching gas is disfavored for at least two reasons. The addition of any gas for a non-etching purpose complicates the optimization process for the etching. A good commercial process requires a wide process window for pressure, temperature, gas flow, etc. to assure uniformity and repeatability in the etching. Almost inevitably, a more complex gas reduces the process window. Furthermore, nitrogen has been observed in a chlorine-based etch process to cause a build up of deposits on the chamber wall. The build up needs to be periodically removed by manual cleaning. Such cleaning reduces throughput, requires operator time, and may itself introduce particles into the chamber.
Chamber erosion in plasma etch reactors is an old problem, and much development work has been devoted to improved and new materials. However, we have found that the conventional available materials exhibit unacceptably high erosion rates in the environment of a high-density BCl.sub.3 and Cl.sub.2 plasma. Several coupons of different materials were tested under the same conditions as given above for the anodization tests. The results are shown in TABLE 1, where the weight loss is measured for standard sized coupons.
TABLE 1 ______________________________________ Material Manufacturer Weight Loss (%) ______________________________________ Alumina Type 1 A 0.112 Alumina Type 2 A 0.256 Alumina B 0.262 Titania C 0.373 Silicon Carbide D 0.239 Silicon Carbide Improved D 0.278 Sapphire 0.393 ______________________________________
These results show that standard techniques of protecting the wall are not adequate for advanced metal etchers.